Many qualitative and quantitative diagnostic self-tests have developed in the clinical field utilizing blood as a sample. Although some of these techniques can be carried out on whole blood, it is necessary with many diagnostic assays to utilize serum or plasma separated from the blood sample to obtain an accurate analysis. Otherwise, the red blood cells or impurities in the blood sample could adversely affect a measurement of either reflected or transmitted light of a diagnostic test relying on either of these measurement techniques.
The most common conventional technique for separating plasma or serum from whole blood is by centrifugation. This technique, however, is time consuming and requires equipment that is not generally available outside of the clinical laboratory. For these reasons, centrifugation is entirely inappropriate for performing diagnostic self-tests by consumers or by professionals in point-of-care facilities.
A number of methods have been devised to avoid these problems. Usually a filtering device is used to separate red blood cells from plasma. Numerous materials have been used to form filters. Paper, fabric, glass or synthetic fibers, and membrane materials having a suitable pore size appear in the prior art.
For example, U.S. Pat. No. 4,816,224 to Vogel discloses a device for separating plasma or serum from whole blood and analyzing the serum using a glass fiber layer having specific dimensions and absorption to separate out the plasma from the whole blood for subsequent reaction. Among the problems of using glass fibers alone or in an admixture with other fibers is the slow rate of penetration by whole blood and the tendency to clog easily with practical yields of plasma usually being less than 25%. Furthermore, only small amounts of plasma or serum are entirely devoid of red blood cells or hemoglobin contamination which could interfere with subsequent quantitative measurements.
Another example is U.S. Pat. No. 4,753,776 to Hillman et al. which disclose a method of separating plasma from blood using a capillary force to drive the blood through fibers to a reaction area. Once the reaction area is saturated, the capillary force ceases and the transportation of the blood through the fibers and onto the reaction area stops. Two specific filtering media are disclosed: glass fibers alone or other cellulosic materials fibers using a soluble red cell agglutinin added in free form to the filtration medium. The free form of agglutinin, however, can migrate from the fibers to contaminate the plasma with the red blood cells bound to the agglutinin. Also, a portion of the plasma is unusable because it remains in the filter to fill the void volume created in the bed of fibers. Another problem can be the concentration of the agglutinin. A minimum amount of agglutinin is required to achieve agglutination. Too much agglutinin slows the movement of the plasma.
These prior art methods have proven unsuitable for applications involving diagnostic devices with space and volume restraints. Diagnostic devices also require an efficient method of separating plasma from a minute blood sample, often just one drop, to produce a sufficient volume of plasma to be transported through the assay portion of the device. The time allowed to complete the separation of the blood sample is also important so that the reaction chemistry can be accurately completed and the results are provided in a timely manner for the convenience of the user.
Thus, a need exists in the field of diagnostics for a blood separation filter and method which is sufficiently timely, efficient, and reliable for use in a diagnostic device which permits point-of-care use by untrained individuals in locations such as the home, sites of medical emergencies, or locations other than a clinic.